It will be appreciated that press services maintain a newsphoto transmission network for its member newspapers in which photographs or negatives are sent over a leased telephone circuit by a local transmitter which converts the image to be transmitted into an amplitude modulated audio frequency signal. A receiver at the member newspaper converts the transmitted signal back into a high resolution photograph or negative available for use by the newspapers. When a story occurs in a remote location, the photograph may have to be transmitted on a regular dial-up circuit to a press bureau from which it can be transferred to the photo network. Alternatively, member newspapers may send a photographer to another city and transmit their photographs directly back to their newspaper on a dial-up telephone circuit.
While photojournalists have regularly been provided with portable transmitters which are high resolution picture transmitters that couple directly to the dial-up telephone, due to characteristics of certain new telephone networks, transmissions from many of the lines have had substantial numbers of streaks or drop outs, yielding unuseable pictures at the receiver end of the system. At times, a photographer, or other operator, must try more than ten telephone lines before a line is isolated that can be used to yield a clean transmission. This is, of course, time consuming and can take numbers of hours, resulting in either loss of the pictures or loss of timely reporting of the news via pictorial means.
The problem described above has only recently arisen. Prior to recent changes in telephone networks, it was generally a relatively simple matter to send a facsimile image or other type of image from a local telephone to the newspaper or news service. However, for reasons that cannot at present be explained, considerable difficulty has been experienced utilizing standard equipment for the purposes of transmitting high resolution photographs or negatives from one place to another utilizing commercially available telephone networks, whether alternative networks or those owned and operated by the American Telephone & Telegraph Company.
Spectral analysis of the signals has yielded no apparent reason for the breakup or degradation of the pictures transmitted. Moreover, the difficulties appeared to be more common during time periods when high phone line use would be expected, such as those times corresponding to the times when news events are taking place.
By studying a number of examples of the streaked transmissions, it was originally thought that electronic noise was the source of the problem since it seemed that audio signals generated by the local transmitters were in some way exciting or interfering with telephone company equipment which would respond by generating signals independent of the transmitted signals. However, no consistent explanation for the degradation of the finally received product was available.
It was then found that the problem occurred more frequently when changes from black areas on the original photograph to white areas were rapid or instantaneous. A test pattern resembling a checkerboard was constructed and it was ascertained that this type of rapid transition between a black area and a white area, or vice versa, excited the most amount of streaking when transmitted over certain types of telephone lines. It will be appreciated that in terms of the amplitude modulated signal transmitted over the audio bandwidth of the telephone network that rapid changes in signal amplitude from about -55 DBM to about -10 DBM in a period of a couple of micro-seconds produced the most picture degradation. Thus, it became apparent that there was a need for a solution which would be able to handle and eliminate the effect of any rapidly changing analog AM signal which was within the telephone network bandwidth of approximately 300 Hz to 3,000 Hz.